Many high-precision machined parts are produced in a multi-step process. First the surface is machined, possibly from a casting, to give it the approximate desired dimensions. Then the surface is honed, ground or cut in a finishing operation that removes only a very thin layer of metal to produce the desired precise dimensions and surface finish. Examples of this multi-step process are the face milling of flat surfaces in engine heads and engine blocks and the machining of cylinders in engine blocks. The cutting tools used to perform these operations must be precisely located and aligned. This is particularly important for the finishing tools, because they remove a very thin layer. If they are not properly positioned, they may miss the metal surface entirely in some places leaving only a coarser finish in those places.
A specific case of multi-step machining is the machining of cylinder bores. In the machining process the cylinders of an engine are first machined in one or more boring operations, usually a coarse and fine boring operation. Then the cylinders are finished by going through one or more honing operations until the final finish is achieved. If the machining marks from the boring operation have not been removed in the honing operation, the cylinder may burn oil and generate noise when the engine is running.
In a cylinder bore, if the coarse and fine tools are aligned in the same direction but are not perfectly concentric, a strip along the inside surface of the cylinder could remain coarsely finished after the fine finishing process. If tools are centered on the axis of the cylinder, but are not at precisely the same angle of inclination, a strip of coarse bored surface may extend part of the way down one side of the cylinder and continue on the opposite side of the cylinder along the rest of its length. By mapping the pattern of coarse and fine finished regions it may be possible to determine the type of misalignment that caused the problem so that it could be quickly corrected without producing additional defective parts.
Differences in surface finish can be recognized by inspection by a knowledgeable expert, but human inspection is not totally reliable. An automated inspection procedure for inspecting surface finish for cylinder bores that is faster, less subjective and more reliable than human inspection is desirable. It is also desirable to automatically distinguish between surfaces with different finishes to determine whether the machining operations have been performed as intended.
The use of scattered light to obtain information about surface structure has been studied extensively. A summary of different measurement techniques is given in an article by G. J. Dixon entitled “Light scattering maps surface imperfections” Laser Focus World pages 89–94, November 1998. Much of the work in this area is directed at obtaining detailed surface profiles or values of surface roughness from scattering measurements. These techniques are capable of distinguishing between different surface finishes, but they are not designed to obtain this information sufficiently rapidly over large areas of machined metal surfaces to be used for one hundred percent in-process inspection.
Another technique that is used to infer surface roughness is total integrated scattering. The theory to evaluate these measurements is only valid when the surface roughness is small compared to the wavelength of the probing light, which is not the case in machining processes. Optical Dimensions of Bozeman, Mont. has developed a device to measure surface roughness using scattered light, but it can only provide repeatable values independent of the direction of incidence when the scattering is independent of the orientation of the surface being measured. This will be the case for ground surfaces, not surfaces with a pattern of machining marks. When machining marks or honing scratches are present, light is preferentially scattered in planes perpendicular to these machining marks. Angle resolved scattering techniques have been developed to analyze the scattering patterns produced by these surfaces. Analysis of this data is generally a lengthy, time consuming process that is usually performed only in research labs.
A cost effective apparatus that is capable of inspecting large surface areas rapidly in real time manufacturing processes is desirable.